Copper etching is a crucial step in the processing of multilayer printed circuit boards (MLPCB). Copper sheets, laminated to glass fiber reinforced epoxy board substrates and destined to become inner layer circuitry, are imaged and then etched typically using acid CuCl.sub.2 -based etchant solutions to produce the inner layer circuitry on the substrate. Two or more individual substrates are then bonded with inner layer circuitry being inside of the MLPCB. An example of an intermediate stage of a MLPCB, denoted as 1, is shown in FIG. 1. The intermediate stage comprises boards, 2, of a C stage glass fiber reinforced material, metal conductors, 3, and bonding layers, 4, which may be of a prepreg or B stage material. The intermediate stage may also include metallizations, 5, on the outer surfaces of the boards, which eventually shall be patterned into the outer circuitry of the MLPCB. Typically, alkaline copper etchants are being used for this etching.
The acid cupric chloride etchant is used to produce about 80% of inner layer boards, 85% of the print-and-etch boards, and 90% of the flexible circuit boards manufactured in the United States. The presently used etchant and associated equipment are adequate for relatively simple circuits. However, significant non-uniformities in the etch rate across the boards are observed, with variations in the etch rate across the board ranging typically from 5 to 10% and in some instances up to 30%. This suggests that the processing of these boards requires more stringent control of the copper etching process. In addition, the multilayer boards developed for computer applications are more complex and use smaller conducting lines. Improving the etch uniformity should improve the fine line etching capability.
Numerous attempts were made to improve the uniformity of the copper etching process with CuCl.sub.2 etchant. For example, Isaev and coworkers indicate that addition of metal chlorides increases the etching rate. See V. V. Isaev et al., Zasch, Met., Vol. 13, No. 4, July 1977, pp. 444-445. Improvement in the etching of metals using solutions containing both ferric and cupric chloride by varying the pH, temperature, and salt concentration has also been reported by A. F. Bogenschuetz et al., Chemical Abstracts, Vol. 90, No. 161138z, 1979. It has also been reported that undercutting effects may be reduced by the addition of a mixture of an anionic surfactant with a structure RO(CH.sub.2 CH.sub.2 .dbd.O).sub.n SO.sub.3 R' and a nonionic surfactant of N-alkanolmonocarboxamide, where R' represents either H or an alkali metal and R is a (C.sub.6-15) alkyl group. See A. Tanaka, Chemical Abstracts, Vol. 105, No. 105-106957u, 1986.
However, while some progress was made in improving the etch rate, the need for more stringent control of the etching process and more uniform etching of copper is still present.